Electrophysiological studies will be conducted in rat brain slices to examine the characteristics of excitatory and inhibitory inputs to subthalamic neurons. Recordings in normal rats will be compared to those of rats treated with 6-hydroxydopamine (6-OHDA) and levodopa-treated animals, which will allow insight into Parkinson's disease. Specific aim 1 is devoted to characterization of voltage-dependent currents and effects of dopamine on membrane properties of subthalamic neurons in vitro. These studies will address the hypothesis that prolonged stimulation of subthalamic neurons reduces their activity because of persistent inactivation of voltage-gated calcium currents or desensitization of glutamate receptors. This hypothesis will specifically address how stimulation of this region "deep brain stimulation "may yield a clinical effect. Specific aim 2 will examine the effects of dopamine, ACh, and other transmitters on synaptic currents in both subthalamic and substantia nigra - reticulata neurons. The choice of dopamine and acetylcholine is logical given that these transmitters are likely to have effects normally. There is a specific hypothesis to be addressed, that is, chronic dopamine depletion and levodopa treatment, akin to Parkinonism, would alter receptor pharmacology. The manner that the experiment will address this hypothesis is not as clear as in specific aim 1. Specific aim 3 will identify ionic mechanisms that are responsible for burst firing in subthalamic neurons. Modulation by dopaminergic manipulations will be examined also. The importance of understanding burst firing is that this type of activity is more likely than single discharge to influence target neurons. Understanding the output of the subthalamic nucleus will clearly have relevance for understanding movement disorders and Parkinsonism. Specific aim 4 is based on previous studies showing that dopamine depletion/levodopa treatment both have chronic effects on GABA and muscarinic receptors in the basal ganglia. Thus it is hypothesized that this treatment will influence the synaptic potentials in both the subthalamic and substantia nigra- reticulata neurons. Therefore, rats which are lesioned with 6-hydroxydopamine or treated with levodopa will be examined in vitro for synaptic changes in the subthalamic nucleus and substantia nigra-reticulata.